1. Field of the Invention
The current invention concerns an enhanced fluid operated pinning and cooling assist device and method used for the production of thermoplastic film by the cast film process.
2. Prior Art
It has long been recognized that better contact between the molten web and the chilled casting roll used to produce thermoplastic film by the cast film process results in better web cooling rates, improved production rates, improved properties dependant upon a fast cooling rate, and improved uniformity of gauge of the resulting film. To promote better contact between the molten web and chilled cooling roll numerous devices have been tried, all successful to a greater or lesser degree, to pin the melt curtain to the casting roll and/or eliminate or minimize the layer of air trapped between the melt curtain and the casting roll.
Initially, some forms of electrostatic pinning devices were used to secure the web to the chill roll. In U.S. Pat. No. 3,102,302 a full width electrode for pinning the web to the chill roll is disclosed and claimed, while on the other hand U.S. Pat. No. 3,223,757 discloses and claims an electrostatic devices for pinning the edges of the web to the chill roll. Eventually, however, increasing web speeds revealed shortcomings to the electrostatic pinning devices; as web speeds increased the tendency to entrain air between the web and the chill roll also increased.
To overcome this high-speed air entrainment problem other types of devices were introduced. Typically, these devices move air either at or away from the roll and melt curtain. One of the first such devices was an air knife as shown in U.S. Pat. No. 4,874,571. This device delivers a linear stream of air against the melt curtain at or near the location at which the melt curtain strikes the chilled casting roll, thereby forcing entrapped air from between the melt curtain and the casting roll surface. Additionally, vacuum boxes designed to pull air away from the melt curtain/roll surface interface, i.e. from under the web, as shown in U.S. Pat. No. 4,310,295 were also used. To further improve contact between the web curtain and the chill roll, another device, hereinafter referred to as a soft box, was eventually introduced. The soft box approached the problem of improving contact by directing a diffuse stream of pressurized air over the web curtain. Unlike a conventional air knife, however, which delivers the air stream essentially in a linear pattern, the stream from the soft box is more diffuse and is applied over a significant portion of the roll circumference. See U.S. Pat. No. 5,091,134 for one configuration of what is herein referred to as a soft box.
While the soft box has resulted in improvements in rate of film production and heat transfer related properties of the resulting film such as clarity, it has also inadvertently resulted in a reduction in film gauge uniformity. Specifically, the problem with reduced gauge uniformity appears to be related to flutter introduced into the web in the unsupported span between the die and the chill roll by air escaping from the soft box. It is therefore an object of the present invention to provide an enhanced cooling device for cast film production which will eliminate the problems associated with air discharge from conventional soft boxes. It is still another object of the present invention to provided an improved cooling device for cast film production which will result in higher production rates for cast film than heretofore seen while improving or at least maintaining acceptable gauge performance.
In accordance with the present invention, there is a new cooling apparatus for cast film comprising a first plenum for supplying a diffuse stream of air to the surface of a melt curtain on a casting roll and a second plenum for producing a directional source of pressurized air so located at to discharge pressurized air into the air discharge stream from the upper portion of the first plenum whereby the air discharge from the first plenum does not strike and induce flutter in the melt curtain between the die and the chill roll.